Time-resolved plasmonics in designed nanostructures

Abstract: A metal nanoparticle can be considered as consisting of a base of positive ion cores and a sea of free electrons. When the free electrons are displaced, for example, by an incident electric field, a restoring force acts on the electrons. The electrons may then oscillate back and forth until equilibrium is reached. This oscillation occurs at the natural frequency, or eigenfrequency, of the system. By matching the driving frequency with this frequency, the amplitude (the maximum electron displacement) can be made large - the system is in resonance. This resonance mode is a plasmon. The separation of charge on that small length scale will result in a large field in the vicinity of the nanoparticle. This large field, often oscillating at optical frequencies, on the spatial scale of nanometers, has many potential applications, such as high-resolution microscopy, photo-voltaics, light emission and coherent control. Because of the interest in manipulating light on the nanoscale, particles having their resonances in the optical domain are often used. The collective electron oscillation, when resonantly excited, therefore occurs on the femtosecond timescale. Due to this ultrashort timescale, the dynamics are difficult to follow in time. The spatial confinement of the oscillation to the nanometer scale makes it challenging to also image them. This thesis explores ways of studying the ultrafast dynamics of plasmons spatially and temporally, simultaneously. Two types of experiments are discussed. The first is autocorrelation experiments where the induced and enhanced field is autocorrelated with itself. For one of these experiments, bowtie nanoantennas were manufactured, using the focused ion beam technique. In the second kind of experiment an infrared laser pulse is used to excite the plasmon, and a short attosecond pulse probes it. The work described in this thesis deals with the fabrication of nanostructures and the implementation of attosecond pulse generation schemes suitable for this purpose.